1. Technical Field
The invention relates generally to implantable medical devices. More particularly, the invention relates to an implantable cardiac defibrillator that provides the ability to control the capacitance of the high voltage output stage.
2. Description of the Prior Art
The use of implantable cardioverter/defibrillators (ICDs) to treat cardiac tachyarrhythmias is well known in the prior art. Advances in microelectronics and microprocessors have allowed for reduction in the size and weight of such ICDs, and for the use of more sophisticated techniques in the detection and treatment of such arrhythmias. The two current primary limiting factors in the reduction in size of ICDs are the high voltage capacitors and the batteries. A higher energy density, and thus smaller, capacitor design is disclosed in U.S. Pat. No. 5,230,712 "Method for Producing Multi-Cell Solid State Electrochemical Capacitors and Articles Formed Thereby" to Matthews, which is incorporated herein by reference. Such capacitors allow for output stage high voltage capacitor configurations which have not previously been practical.
Kroll, et al, "Method and Apparatus for Far-Field Tachycardia Termination", U.S. Pat. No. 5,330,509, discusses a two-capacitor system within the context of far-field cardiac stimulation. Kroll discloses the use of a second capacitor to deliver the far-field stimulation pulses, in addition to the pulses provided by the primary capacitor. The defibrillation device, while using two capacitors, does not have the ability to selectively use one or both capacitors for the delivery of a single pulse. Further, the secondary capacitor does not function as a back-up to replace a malfunctioning primary capacitor.
As ICDs are used to treat an expanding variety of arrhythmias, the consequences of an ICD failure are of increasing significance. One of the components of an ICD that has a low, but real, possibility of failure is the high voltage capacitors of the output stage. Current systems monitor capacitor charge times to allow identification of a degraded capacitor. It would be a significant advance in the art to provide an implantable cardiac defibrillator which could replace a substandard capacitor with a back-up capacitor. Current practice is to remove and replace an ICD if there has been an HV capacitor degradation or failure. Because of this, very expensive capacitors are used in ICDs and they are subjected to extensive quality control testing to ensure that there is no failure. However, such extreme measures might not be necessary in a system having backup or redundant capacitors, since redundant, standard off-the-shelf capacitors might actually provide a higher system reliability than is provided by a single super-premium capacitor.
An additional issue associated with the HV capacitors of an ICD is the amount of time it takes to charge the capacitors. The charge time is typically about 10 to 15 seconds and is governed by the formula EQU V=V.sub.app (1-exp (-t/RC))
where V is the target voltage, V.sub.app is the applied voltage, R is the effective resistance of the capacitor and C is the capacitance of the capacitor. It can be seen that the charging time is directly proportional to the capacitance. Since it has been found that defibrillation shocks are most effective when delivered as quickly as possible following the detection of fibrillation, the shorter the charge time for the capacitors the more effective the defibrillation therapy.
For each patient receiving an ICD, the surgeon performs a series of tests at the time of implant to determine the defibrillation threshold (DFT) for that patient. The DFT is a measure of the minimum energy required to defibrillate that patient's heart. This value may vary significantly from patient to patient and may change over time for a given patient. Once the DFT is determined, the ICD is programmed to deliver defibrillation shocks of an energy with a safety margin above the DFT. The energy stored by the HV capacitors is a function of the capacitance and voltage according to the formula EQU E=1/2CV.sup.2
A significant fraction of this energy from a charged capacitor is delivered to the patient depending on the programmed time duration of the shock. Prior art systems have a fixed capacitance and thus, for a given energy delivery requirement, the voltage to which the capacitors are charged controls the energy in the shock delivered to the patient and this energy is a linear function of the capacitance of the capacitor. The charge time for the HV capacitors is thus fixed for a given final voltage (assuming a constant battery supply voltage.) It would be advantageous to provide a system allowing a reduced charge time for the HV capacitors in cases where the patient's DFT is low enough so that it isn't necessary to utilize the total available system capacitance to deliver sufficient defibrillation energy to the heart.
A typical operational sequence to which an ICD may be programmed is to provide a first defibrillation shock at 500 volts, a second shock at 700 volts if the first shock fails to defibrillate the heart and then two additional shocks at 750 volts if the preceding shock in ineffective. Each of the increases in voltage significantly increases the energy delivered since the energy is a function of the square of the voltage. However, in some cases these shocks still do not defibrillate the heart. It would be beneficial to have a way to provide additional energy delivery capability in the event of failure of the programmed shock sequence to effect defibrillation.
It is therefore an object of the invention to provide an ICD with a high voltage output stage having a selectively variable capacitance.
It is a further object of the invention to provide an ICD with back-up high voltage capacitors to replace degraded capacitors.
It is another object of the invention to provide an ICD wherein back-up high voltage capacitors are used to provide a maximum energy defibrillation shock in the event lower energy shocks fail to defibrillate the heart.